This invention relates to a switching circuit which closes a power circuit; and more particularly to a switching circuit in which the initializing signal is isolated from the power circuit that is to be switched and where current limiting is provided in the switching circuit.
In most power circuits, the theory of controlling the closing and opening of a circuit is the same regardless of the magnitude of the voltage and currents to be switched. However, the techniques used in different circuit applications may vary considerably. The present invention is directed to the opening and closing of circuits which normally have relatively small current (in the order of 15-80 milliamperes) and relatively low voltage (in the order of 24 to 80 volts). Simple relay circuits may be used in many applications to effect the opening and closing of circuits in which voltages and currents of these magnitudes are present. More recently, switching transistors have been developed which will also switch powers of this magnitude. However, such transistor circuits do not readily permit the isolation of the initializing signal from the power circuit, nor do they include a current limiting arrangement for reducing the power which is permitted to pass through the closed power circuit. The requirement for isolation is important in circuits in which the longitudinal balance is to be maintained, and such a requirement is necessary for many communication circuits. The requirement for minimizing power consumption is important in present day electronic circuits, particularly where such circuits are under control of a microprocessor.
Although the present invention is described in relation to its application to a telephone signalling system, it is to be understood that the invention may be employed in many other applications and, therefore, is not to be limited to such applications, but only should be limited by the appended claims.